(1) Field of the Invention
This invention relates to a sputtering target used for a transparent electrically conductive thin film.
(2) Description of the Related Art
An indium tin oxide (hereinafter abbreviated to "ITO") thin film is characterized as possessing a high electrical conductivity and a high transmittance and in that precision working thereof can be easily conducted, and therefore, the ITO thin film is widely used as, for example, a transparent electrode for a flat panel display device, a window material of a solar cell, and an antistatic conductive film. In recent years, flat panel display devices including a liquid crystal display device have become large-sized and more precise, and therefore, there is an increasing demand for an ITO thin film used as a transparent electrode for a flat panel display device.
The methods for making an ITO film are classified into two types, i.e., a chemical film-deposition method such as a spray-thermal decomposition method or a chemical vapor deposition method, and a physical film-deposition method such as an electron-beam deposition method or a sputtering method. Of these, the sputtering method is widely used in various fields because an ITO thin film having a large size and reliable performance can be easily fabricated.
In the sputtering method for fabricating an ITO thin film, sputtering targets are used which include a metal alloy target composed of metallic indium and metallic tin (this metal alloy target is hereinafter abbreviated to "IT target") and a composite oxide target composed of indium oxide and tin oxide (this composite target is hereinafter abbreviated to "ITO target"). The ITO target is more popularly used than the IT target because changes with time of resistance and transmittance of the ITO thin film fabricated by using the ITO target are minor and the film-fabricating conditions can be easily controlled.
When an ITO target is continuously sputtered in a mixed gas atmosphere composed of argon and oxygen, a black deposit called a "nodule" is formed on the target surface with an increase of the integrated sputtering time. The black deposit is believed to be a lower oxide of indium and is formed in the periphery of the erosion area of the target. It is known that the nodule deposition occasionally leads to arcing upon sputtering and causes generation of undesirable particles. Consequently, when sputtering is continuously carried out, contaminants are found in the formed thin film, which lead to reduction of the yield of flat panel displays of liquid display devices and cause failure of elements. The failure of elements is serious in the field where a high precision is required, such as flat panel displays.
To remove the contaminants from the thin film, a cleaning operation is conducted at regular intervals. This leads to reduction of productivity. It is therefore eagerly desired to provide an ITO target wherein the nodule deposition is minimized.
To provide such ITO targets wherein the nodule deposition is minimized, a proposal has been made in Japanese Unexamined Patent Publication No. 5-148635 wherein powdery indium oxide and powdery tin oxide are compression-molded together, the molded mixture is sintered in an atmosphere having an oxygen partial pressure of at least 1 atm, and the sintered product is subjected to machine finishing by a conventional procedure whereby an ITO target having a center line, average surface height (Ra) of not larger than 0.5 .mu.m is obtained. However, it now has been found that there is a great variation among the thus-produced ITO targets in the amount of nodules deposited when the ITO targets having an Ra of not larger than 0.5 .mu.m, produced by the proposed method, are sputtered under the same conditions.